Heads for reading data from storage media convert modulation of a reflected optical beam from an optical disk or magnetic flux variations from a magnetic disk or tape into analog electrical signals. The analog information signals are subsequently processed through an apparatus, such as a qualified peak detector with an envelope follower used to derive a threshold for peak qualification, and converted into digital signals representative of the data read from the medium. In a conventional envelope follower, the information signal is applied to a capacitor through a diode. When a pulse, representing information from the storage medium, occurs having an amplitude greater than the voltage across the capacitor plus the forward bias of the diode, the diode is turned on, charging the capacitor.
After the pulse passes, the capacitor discharges at a predetermined rate (dependant upon the time constant of the capacitor but slow relative to the frequency of the information signal) until the next pulse charges the capacitor. Thus, the voltage across the capacitor forms the upper boundary of an "envelope" approximating the maximum amplitude of the analog information pulses (minus one diode drop); the lower boundary of the analog information signals can be a predetermined DC base level, such as provided by a DC restore circuit, or could be followed by another envelope follower. To allow low amplitude signal to be detected and processed as an information pulse, the envelope voltage is reduced, such as by a voltage divider, to a threshold level. Both the information signal and the threshold voltage are input into a comparator which outputs a digital signal only if the amplitude of a pulse in the information signal exceeds the threshold voltage. This is commonly used as the "qualifier" of a qualified peak detector.
Although amplitudes of the information pulses making up the analog signals are generally within a particular range, the information signals from the read head can contain pulses of particularly high amplitude ("glitches" or defects) outside the normal range. For example, if a pit in an optical disk is elongated or otherwise not properly formed, or if a physical defect exists in the optical media surface, the resulting information signal can also be defective and contain a glitch. Information signals generated by magneto-resistive heads for reading magnetic tape can contain singularities caused by thermal spikes. Not only are such defects and spikes processed as information pluses, they can also charge the capacitor to such an extent that the threshold voltage, related to the envelope voltage, remains greater than the maximum amplitude of one or more subsequent valid information pulses. Consequently, in a condition known as defect propagation, those valid pulses will not be detected and information will be lost. Moreover, current through portions of the envelope follower can exceed the maximum specified current thereby reducing component life or even destroying components.
Prior attempts to reduce defect propagation have included clipping the information signal or limiting the envelope to prevent excess charging of the capacitor but have not been wholly satisfactory. For example, an approximately 2:1 headroom should be provided between the normal envelope and the voltage which clips the threshold to allow normal envelope following operation of a low AC coupling pole for information signal having a large DC content. However, defect propagation can adversely impact proper peak detection. Additionally, a common conventional design employing a "diode into a capacitor" type envelope follower described above is non-linear due to the diode characteristics.